1. Field of the Invention
The invention relates to the coloring of cellulosics, inclusive of color printing techniques, and more particularly relates to the coloring of cellulosics with chemically reactive dyes to obtain bleed resistant, colored cellulosic articles.
2. Brief Description of the Prior Art
The U.S. Pat. Nos. 1,732,540; 1,805,013; 1,863,813; 1,871,647; and 1,871,769 are representative of descriptions of prior art methods and practices for preparing so-called color fast, colored cellulosic articles. In spite of the known art for preparing colored cellulosic articles, there is a considerable amount of commercially available articles such as crepe paper, colored with dyes, that "bleed" when put into contact with water. By "bleeding" we mean that the color leaches out of the paper and may stain whatever the wet paper comes into contact with. This effect is considered undesirable for many uses of crepe paper. Apparently the known prior art methods of preparing color fast dyes cellulosics have not been found practical, sufficient or efficient for present day practices.
Prior to our invention, a class of dyes (reactive dyes for cellulosics) were known, which react with free hydroxyl groups on cellulosic fibers to cause a coloring thereof. However, the fibers must be free of contaminants such as starch, which would also react with the dye to form colored, insoluble particles on the fiber surface. The starch/dye reaction product is readily washed off of the fiber surface, resulting in a waste of dye reactant. Another disadvantage is found in the long reaction times needed to affect a coloration, i.e.; 20 to 30 minutes at elevated temperatures. In another way to use such dyes (to dye paper, for example), this class of reactive dye may be added to the paper pulp in the beater stage to permit reaction with cellulose over a prolonged period of time. The dyed pulp is then made into a paper sheet. The resulting colored paper is color fast (bleed resistant). The disadvantage of this approach is that a considerable amount of a given colored paper has to be made at one time (for economic reasons), and thus an inventory must be maintained until demand depletes it. This is a costly procedure.
By the method of this invention the reactive dyes may be first reacted with a cross-linking agent such as a melamine-formaldehyde resin. The resulting colorant compound is then used to impregnate preformed cellulosic articles such as crepe paper and then further reacted with the paper to chemically crosslink the dye to the crepe paper. The result is a colored crepe paper sheet which resists "bleeding" when placed in contact with water. The method of the invention is economical and may be used in a conventional plant for commercial coloration without purchasing new equipment or modifying that already in existence. Reaction times required are relatively short, which is an unexpected advantage. The method of the invention may also be carried out at relatively low temperatures, rapidly. Thus there are operating and economic advantages. The coloring approach described above may be used on a wide variety of woven and non-woven disposable cellulosic items where non-bleeding is a useful factor (for example, tissue paper, towelling, rayon sheets, crepe paper and the like).
Our observations made while coloring crepe paper indicated that the color dispersions used in the method of the invention can be used for printing on the surfaces of cellulosic articles. Prior to the invention, a considerable amount of color printing on low cost substrates was accomplished with either resin bonded pigments or solvent based inks. Both of these systems operate on a common principle, that is, attachment of the colorant to a substrate using a binder. Resin bonded systems are comprised of highly dispersed water insoluble pigments in water based emulsions. An acrylic emulsion is most commonly used in such systems. Preparation of the print paste involves addition of suitable thickeners to achieve good definition of the print, then drying to coalesce and adhere the pigment particles to the substrate. Solvent based systems are comprised of solvated dyes and lacquers. Preparation of the print paste involves addition of suitable thickeners to achieve good definition of print, then drying to evaporate the solvent and permit the lacquer to hold the colorant on a substrate. Both of the above described systems are expensive. For the printing of low cost items such as paper, paperboard, gift wrapping paper, textiles or other disposable items, the method of the invention is advantageous since it adds little to the overall cost of manufacture. The method of the invention is not limited however to the coloring or printing of low cost disposable cellulosics but may be used to color even more expensive articles such as textile fabrics, including woven, non-woven, knitted, pile and velvet fabrics, webs and yarns. By the method of the invention the printing of cotton fabrics is particularly advantageous. For example, the cotton fabric need not of necessity be pre-washed to remove starch contaminants.
By the method of our invention a particular class of dyes referred to as "reactive dyes for cellulosics", which will react with cross-linker chemicals containing, for example, amino or hydroxyl groups, are used to form reactive colorant molecules which may be used as a coloring or a printing medium. The advantages are clear in that the water-based system used in the method of the invention has produced a good definition of print. This is of economic importance since currently used organic solvent based systems present occupational and fire hazards and problems of environmental pollution.
In addition, an inert flame retardant may be added to the print pastes or coloring mixtures used in the method of the invention to render the cellulosic articles non-flammable.